This invention relates to a moving apparatus for a load, and more particularly to such an apparatus capable of balancing the load by the torque of a motor.
An apparatus of this character has already been proposed by Applicant and is described in U.S. Pat. No. 3,841,605. The prior art apparatus permits the operator to use only a slight manual effort to handle a heavy load freely as if it were weightless. This operational advantage must be weighed, however, against the disadvantages of the necessity of an uninterrupted power supply to the motor and the use of highly complex control circuitry.
The continuous flow of current is undesirable because it can have adverse effects on the life of the motor and other electric components and circuits and accompanies an accordingly high power consumption.
A possible solution to this problem is to apply a mechanical brake to the lift drive and cut off the power supply to the motor while the load is being held in a suspended state in conformity with the zero-speed designation.
The mechanical brake is to be used need not be novel; it may be of any type commonly in use with electric hoists. The brake is taken off when a push button for lifting or lowering is pressed and is applied as soon as the button is released. This type of brake has the following shortcomings:
The timing relationship between the mechanical brake and motor when the apparatus is to be stopped is such that, in the process of stopping, the mechanical system including the motor and the load will continue to move by inertia and will not allow the actual speed of the apparatus to drop to zero as soon as a zero-speed signal is given. In other words, the apparatus will not stop at once. Nevertheless, the mechanical brake is applied instantaneously upon receipt of the zero-speed signal, and therefore the brake must absorb all the inertial energy of the mechanical system. Thus despite the speed control function with which it is provided, the apparatus is suddenly forced to a stop the moment the speed designation is set to zero. Consequently, the operability of the apparatus is seriously affected and the brake must be designed to have a more than necessary braking capacity in order to absorb the impact energy that is produced at each stop of the apparatus. In addition, the life of the brake is shortened.
On the other hand, it must also be noted that, in the case of an ordinary electric hoist, the power supply to the motor is cut off as soon as the push button is released and therefore it is highly dangerous if the mechanical brake fails to operate immediately. A slight delay in the timing for brake application would cause unintended fall of the load.
In the apparatus according to the above-mentioned prior U.S. Patent, the motor will continue to generate sufficient torque (servo-balanced) to hold the load in suspension even when the speed is set to zero. If in this case a mechanical brake is applied, it will no longer be necessary to hold the load by the torque of the motor and it will become possible to disconnect the motor from its power source.